1. Field of Invention
The present invention relates to toner compositions having improved properties that are provided by improved surface additives. More particularly, the present invention relates to toner and developer compositions where the toner particles have an improved treated alumina surface additive.
2. Description of Related Art
In electrophotography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the photoconductor to a support, such as transparency or paper. This imaging process may be repeated many times.
Various toner compositions for such a printing system are well known in the art, and have been produced having a wide range of additives and constituent materials. Generally, however, the toner particles include a binding material such as a resin, a colorant such as a dye and/or a pigment, and any of various additives to provide particular properties to the toner particles.
One type of additive that is commonly used in toner compositions is a surface additive. The surface additive can be incorporated for any of various reasons, including for providing improved charging characteristics, improved flow properties, and the like.
For example, toner compositions with certain surface additives, including certain silicas, are known. Examples of these additives include colloidal silicas, such as certain AEROSILS like R972(trademark) available from Degussa, metal salts and metal salts of fatty acids inclusive of zinc stearate, aluminum oxides, cerium oxides, and mixtures thereof. Generally, such additives are each present in an amount of from about 1 percent by weight to about 5 percent by weight, and preferably in an amount of from about 1 percent by weight to about 3 percent by weight, of the toner composition. Several of the aforementioned additives are illustrated, for example, in U.S. Pat. Nos. 3,590,000 and 3,900,588, the entire disclosures of which are incorporated herein by reference.
Also known are toners containing a mixture of hexamethyldisilazane (HMDZ) and aminopropyltriethoxysilane (APTES). Disadvantages associated with these toners may include, in certain instances, a low, relatively unstable triboelectric charge, and high relative humidity sensitivity. Further, disadvantages of toners containing as surface additives HMDZ include, for example, unstable triboelectric charge, relative humidity sensitivity, and low charge/wrong sign toner as measured by a charge spectrograph.
Developer compositions with charge enhancing additives, which impart a positive charge to the toner resin, are also known. For example, U.S. Pat. No. 3,893,935 describes the use of quaternary ammonium salts as charge control agents for electrostatic toner compositions. U.S. Pat. No. 4,221,856 discloses electrophotographic toners containing resin compatible quaternary ammonium compounds in which at least two R radicals are hydrocarbons having from 8 to about 22 carbon atoms, and each other R is a hydrogen or hydrocarbon radical with from 1 to about 8 carbon atoms, and A is an anion, for example sulfate, sulfonate, nitrate, borate, chlorate, and the halogens, such as iodide, chloride and bromide. Similar teachings are presented in U.S. Pat. Nos. 4,312,933 and 4,291,111. There is also described in U.S. Pat. No. 2,986,521 developer compositions comprised of toner resin particles coated with certain finely divided colloidal silica. According to the disclosure of this patent, the development of electrostatic latent images on negatively charged surfaces is accomplished by applying a developer composition having a positively charged triboelectric relationship with respect to the colloidal silica.
Also, there is disclosed in U.S. Pat. No. 4,338,390, the entire disclosure of which is incorporated herein by reference, developer compositions containing as charge enhancing additives organic sulfate and sulfonates, which additives can impart a positive charge to the toner composition. Further, there is disclosed in U.S. Pat. No. 4,298,672, the entire disclosure of which is incorporated herein by reference, positively charged toner compositions with resin particles and pigment particles, and as charge enhancing additives alkyl pyridinium compounds. Additionally, other patents disclosing positively charged toner compositions with charge control additives include, for example, U.S. Pat. Nos. 3,944,493; 4,007,293; 4,079,014; 4,394,430 and 4,560,635, which illustrate a toner with a distearyl dimethyl ammonium methyl sulfate charge additive. Surface additives, such as silicas like AEROSILS, may be incorporated into the toners of these patents.
Moreover, toner compositions with negative charge enhancing additives are known, as described, for example, in U.S. Pat. Nos. 4,411,974 and 4,206,064, the entire disclosures of which are incorporated herein by reference. The ""974 patent discloses negatively charged toner compositions comprised of resin particles, pigment particles, and as a charge enhancing additive ortho-halo phenyl carboxylic acids. Similarly, there are disclosed in the ""064 patent toner compositions with chromium, cobalt, and nickel complexes of salicylic acid as negative charge enhancing additives.
U.S. Pat. No. 4,404,271 describes a toner that contains a metal complex where the metal can be chromium, cobalt or iron. Additionally, other patents disclosing various metal containing azo dyestuff structures wherein the metal is chromium or cobalt include U.S. Pat. Nos. 2,891,939, 2,871,233, 2,891,938, 2,933,489, 4,053,462 and 4,314,937. Also, in U.S. Pat. No. 4,433,040, the entire disclosure of which is incorporated herein by reference, there are illustrated toner compositions with chromium and cobalt complexes of azo dyes as negative charge enhancing additives. Other charge enhancing additives include those illustrated in U.S. Pat. Nos. 5,304,449, 4,904,762, and 5,223,368, the entire disclosures of which are incorporated herein by reference.
Despite the broad range of additives that have been used in formulating toner compositions, there is a continued need in the art for improved toner compositions that provide improved results and improved image quality.
For example, U.S. Pat. No. 6,124,071 discloses toner compositions including a polymer and titanium oxide dihydroxide of the formula xe2x80x94Oxe2x80x94Ti(OH)2. The titanium oxide dihydroxide is a charge additive. U.S. Pat. No. 6,087,059 describes a toner comprised of resin, colorant and a surface additive mixture comprised of two coated silicas, and a coated metal oxide. U.S. Pat. No. 6,017,668 discloses a toner composition comprising resin, colorant, and a surface additive mixture of a magnetite and a polyvinylidene fluoride. U.S. Pat. No. 6,004,714 discloses a toner comprised of binder, colorant, and a silica containing a coating of an alkylsilane, including polyalkylsilanes such as decylsilane. Each of the toner compositions are disclosed as providing a number of advantages, such as excellent triboelectric charging characteristics, substantial insensitivity to humidity, superior toner flow through, stable triboelectric charging values, and wherein the toners enable the generation of developed images with superior resolution, and excellent color intensity.
Despite these and other various toner composition formulations, there remains a need in the art for improved toner compositions. For example, although various of the above-described surface additives have provided improved printing and processing characteristics, some of the additives possess certain drawbacks that preferably should be overcome to provide still further improved performance results.
For example, although titanium dioxide (titania), either by itself or in surface treated forms, has been widely used in the art, such a surface additive introduces several problems. In particular, both treated and untreated forms of titania tend to reduce the triboelectric charge of the toner composition. In addition, the titania additives have a higher refractive index (about 2.4 to 2.5) as compared to the polymer resins used to form the toner particles (about 1.5 to 1.7). As a result, the titania causes a higher light scattering, which hurts color toners in both the reflection and transmission modes. Based on both of these drawbacks, the loading amount of the titania must be reduced as much as possible, as higher loadings only exacerbate the problems.
The present invention addresses these problems by using, as a toner particle surface additive, a treated Al2O3 (aluminum oxide or alumina). Use of the treated alumina provides significant benefits to the toner compositions. Unlike titania, the treated alumina does not cause a reduction in the triboelectric charge of the toner composition. Furthermore, because alumina has a lower refractive index (about 1.7), which more closely matches the refractive index of the polymer resins used to form the toner particles, the use of treated alumina allows for a higher loading of the material, without increased light scattering.
In particular, the present invention provides a toner composition comprising:
a binder,
a colorant, and
a treated alumina as a surface additive,
wherein said treated alumina comprises alumina particles treated with a treatment agent.
The present invention also provides developers including such a toner composition, and methods for making such a toner composition.